This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the use of fast imaging with steady precession (FISP) and with motion compensated spiral k-space trajectories.
In cardiac imaging using MRI techniques, rapid acquisition of MRI signals is desired. MR ventriculography is often performed in a gated mode using conventional gradient echo 2DFT techniques. These techniques often suffer from inadequate signal and inadequate contrast between blood and myocardium. Recently, MR scanners have been including a modification of these techniques using steady-state free precession (SSFP) or “FISP” techniques, which refocus all gradient lobes between excitations to preserve transverse magnetization for a longer time. These techniques lead to increased signal and increased blood-myocardium contrast, but cardiac-gating and breath-holding are still required to get good spatial and temporal resolution, because of the inherent inefficiencies of 2DFT scanning.
Fluoroscopic ventriculography is advantageous because cardiac gating and breath-holding are not needed and because it can be used in a real-time interactive mode, similar to echocardiography. Fluoroscopic ventriculography requires an efficient scanning technique, such as spiral scanning. However, conventional spiral ventriculography sometimes suffers from inadequate contrast between blood and myocardium and inadequate signal.
It would be advantageous to use spiral k-space scanning techniques to improve image contrast. However, current spiral scanning techniques use a repetition time (TR) of 25-40 ms, because of long spectral-spatial excitation pulses used for fat suppression and because longer readouts allow for increased scan efficiency. On the other hand, FISP techniques require short repetition times, on the order of 2-6 ms, since with longer repetition times image banding occurs due to field inhomogeneity. Thus, spiral ventriculography and FISP techniques would appear to be incompatible.
The present invention is directed to the practical use of FISP techniques and spiral k-space scanning in motion compensated cardiac imaging.